Method of producing sheets and article to practice such method

ABSTRACT

There is disclosed a method of reducing a billet of stainless steel containing about 2% of boron without edge cracking of the billet and without loss of the boron from the billet. The billet is coated with zirconia and enclosed in a jacket of mild steel and heated to about 1100*C. and rolled to the desired thickness. The zirconia prevents the billet from adhering to the jacket. The stainless steel is produced by melting rods of a standard stainless steel, for example AISI 304L, with a mild-steel can containing an appropriate quantity of boron powder.

United States Patent [191 Miller [111 3,827,264 [451 Aug. 6, 1974 METHOD OF PRODUCING SHEETS AND ARTICLE TO PRACTICE SUCH METHOD [75] Inventor: Gary E. Miller, Monroeville, Pa.

[52] US. Cl 72/46, 29/424 [51] Int. Cl B21b 45/00, B23p 17/00 [58] Field of Search 29/423, 424, 473.5, 474.3,

[56] References Cited UNITED STATES PATENTS 1,962,598 6/1934 Keen 75/1284 2,111,278 3/1938 Charlton. 75/l28.4 2,479,097 8/1949 Buchanan 75/1284 2,653,494 9/1953 Creutz 29/424 2,750,283 5/1956 Loveless 75/1284 3,122,828 3/1964 Havel 29/424 3,139,681 7/1964 Goslee et al. 29/474.3 3,160,951 12/1964 Markert et a1 29/474.3 3,275,426 9/1966 Rowady 75/1284 3,301,668 l/l967 Cope 75/1284 3,352,666 11/1967 Foster et al 75/1284 3,354,538 11/1967 Cadden et al 29/423 FOREIGN PATENTS OR APPLICATIONS 218,375 ll/l956 Australia 75/1284 OTHER PUBLICATIONS The Making, Shaping and Treating of Steel, 1957 edition, page 386.

Primary ExaminerCharles W. Lanham Assistant Examiner--E. M. Combs Attorney, Agent, or Firm-Coleman R. Reap [5 7] ABSTRACT There is disclosed a method of reducing a billet of stainless steel containing about 2% of boron without edge cracking of the billet and without loss of the boron from the billet. The billet is coated with zirconia and enclosed in a jacket of mild steel and heated to about 1100C. and rolled to the desired thickness. The zirconia prevents the billet from adhering to the jacket. The stainless steel is produced by melting rods of a standard stainless steel, for example AISI 304L, with a mild-steel can containing an appropriate quantity of boron powder.

6 Claims, 8 Drawing Figures SIDE RAIL (1.12"): 1.|2"x I8") COVER PLATE (12 GAUGE x 7"x18") PAIENTEUAUB 6:914

SHEET 1 0F 2 FIGJ COVER PLATE PATENTEDAUB m 3.827.264

SHEET 2 0F 2 MELTI-PLATES-ENDZ 3f MELT l-PLATEG-ENDI 'g-3e METHOD OF PRODUCING SHEETS AND ARTICLE TO PRACTICE SUCH METHOD This invention relates to the art of reducing metallic bodies from a greater thickness to a substantially smaller thickness and has particular relationship to such reduction of stainless steel containing substantial quantities of boron, typically in the fabrication of sheets for nuclear reactor control rods.

Control rods are typically composed of plates fabricated into an elongated structure of cruciform cross section. To control the reactivity of a reactor the cruciform structure is slid in and out of the reactor body. Of the elements, which are readily usable for control rod purposes, boron, and specifically, boron 10 (B10) has a high absorption cross section, a b s. It is then desirable that control rods be formed of a material or alloy containing substantial quantities of boron l0 and it is an object of this invention to provide plates for control rods of such a material. Specifically, stainless steel lends itself readily to use in the corrosive surroundings of a reactor and it is an object of this invention to provide such plates of a stainless steel containing substantial quantities of boron 10. The content of B in naturally occurring boron is only 18.83%. For control rod use it is necessary that the enrichment of B10 be about 90%. Such enriched boron is available but it is costly. Typically, the enriched boron is supplied in the form of a fine powder costing between about $4.00 and $6.00 per gram. In the interest of economy it is essential that in the producing of the alloy and in the fabrication of plates for the control rods loss of B10 be minimized and it is an object of this invention to accomplish this desideratum.

Typically, the stainless steel alloy for producing control rods contains up to 2.3 or 2.5% B10 and is produced in the form of billets or slabs having a thickness of about 1.3 inches; typically 1% inches thick X 5 inches wide X 12 inches long. It is necessary that the slabs be reduced to plates or sheets having a thickness of about .350 inches. Typically, sheets or plates having the following dimensions are required.

Thickness 0.312" 1.003" Width 4.125" .000" .015" Length 36:00" .000" .030"

The maximum TIR (Total Indicated Runout) is .010 inch over the 36 inches length. Attempts have been made to produce these plates by rolling the slabs directly in a rolling mill but these attempts have not prorolling was carried out. In a nuclear fabrication shop the boron is deposited on materials being prepared for use in reactors. Such materials have a tolerance of only 1 or 2 parts per million of boron and the deposit of boron on their surfaces would poison the reactors, further the depletion of the boron resulted in gross inhomogenieties in the boron distribution.

It is an object of this invention to overcome the above-described difiiculties and to produce boroncontaining stainless steel sheets by reduction of slabs without spread of contaminating boron throughout the reduction shop, which sheets shall be homogeneous in boron distribution, shall not be cracked at the edges and shall not have surfaces depleted of boron. Typically the homogeniety shall be better than 1'5 relative boron content in analyses of specimens taken from different parts of the sheet.

While this object is in the practice of this invention achieved for stainless steel containing B 10, it is applicable to stainless steel containing boron of other isotopic composition and the fabrication of such steel containing any such composition of boron is within the scope of this invention.

This invention arises from the realization that the cracking and the depletion of boron at the surfaces are caused by the reaction of the slab material with the oxygen in the atmosphere. The boron in the alloy is in the form of complex borides and the borides react with air to produce the harmful effects on the steel. The boron is also released from the steel probably in the form of fine oxide particles to produce contamination of the shop.

In accordance with this invention the reduction is carried out with the stainless steel confined in cladding of mild carbon steel. Each slab is coated with a wash of a material, typically, zirconia, which prevents the slab surface from sticking to mild steel and is then sealed in a pocket of mild steel. The composite pack is then reduced by repeated rolling to the desired thickness. Prior to each rolling operation the pack is heated to 1 125C i C. It is essential that the temperature shall not exceed about 1165C. during the heating because the boron-containing stainless steel exhibits hotshortness and deteriorates above 1165C. During the rolling it is essential that the temperature shall not fall below about 1075C. because at this temperature the boron-containing steel becomes brittle and cannot be rolled. The pack according to this invention has the additional advantage that it retains the heat longer than a slab of the stainless steel and thus facilitates the rolling. The cladding also constrains the slab and prevents edge cracking.

ln producing boron containing stainless steel, typically, type AISI 304 stainless steel, is melted with a reqduced satisfactory sheets. The sheets produced were i i quantity f 90% i h d B10 powder i a Cracked at g B pp suffahes 0f the uum furnace. Where low carbon is essential type 304L She etS fabrlcaled thls manner h regions from steel may be used. The following Table I presents the whlch the 1310 as d plet x n g to a ep h of nominal composition of the alloy elements in percent about -070 Inches fI'Om h Sur faC6- BOIOI) bolldes of 304 steel and the actual analyzed composition in Was released from the Slabs during the rolling and 3 alloy elements of such steel used in the typical practice deposited on surfaces throughout the shop where the of this invention.

TABLE I C Mn P S Si Ni Cr Mo Co Cu Nominal .011 2.00 .045 .030 1.00 8.00 18.00

12.00 2000 Actual .072 1.51 .021 .013 .62 8.38 18.43 .28 .10 .36

The remainder for each row is iron.

Typically, the boron which is melted with such steel has the composition given in the following Table IA.

TABLE IA NBL-l 90.83

NBL-3 91.50

NBL-4 91.73

Lot No.

Total Boron Niagara Analysis Total Boron NUMEC Analysis Isotopic 8-10 Niagara Analysis H O Ti The boron is supplied in the form of a powder which would be partially dissipated during the evacuation of the furnace if charged directly into the furnace. In accordance with an aspect of this invention, the boron powder is enclosed in a thin can of mild steel and the can is placed in the furnace. Typically, about 140 pounds of 304 steel and about 3 pounds of boron powder are deposited and the effect of the mild steel of the can is negligible. Specifically, the charge material consisted of certified 304 stainless steel 3 inch diameter bar, and 90% purity-90% enriched B-lO produced by the boron facility at Niagara of Nuclear Materials And Equipment Corp. (NUMEC). Two of the five supplied lots were analyzed by NUMEC for total boron prior to the as-supplied enriched boron and Table I, the certified analysis results of the virgin 304 stainless. Scrap resulting from turning or from rejected plates is remelted and converted into useful alloy.

The virgin boron powder micron particle size) was weighed to the nearest 0.1 gram encapsulated in a 10 mil mild steel can and placed in the bottom of a 200 lb. steel capacity commercial grade MgO crucible. The virgin stainless and/or scrap was weighed to the nearest 0.1 lb. and placed on top of the encapsulated boron. Although the crucible had a 200 lb. stainless steel capacity, the low bulk density of the virgin boron limited the charge to approximately lbs.

Table 11 below shows the make up of Melts Nos. 1 through 21 produced.

TABLE II (All wgts. in grams) Charge Charge Wgt Wgt Wgt Wgt Wgt B- Wgt Melt Composition Composition Virgin Virgin Alloy B in 10 in Virgin Charge No. w/o w/o B-lO B B-10 Scrap Scrap Scrap S.S. Wgt

The following Table III presents the actual analyzed alloy composition of typical melts produced by melting the stainless steel having the composition shown in the second column of Table l and the boron having the above tabulated composition.

TABLE III For a better understanding of this invention, both as to its organization and as to its method of operation, together with additional objects and advantages thereof, reference is made to the following description taken in 5 connection with the accompanying drawings.

For each row the remainder is iron. There was no analysis for Mo, Co and Cu.

The melt is cast in molds to produce billets or slabs having a thickness of about 1.3 inches. The billets are then sandblasted, trimmed and coated with the adhesion-preventing wash coating which may be zirconia.

The coated slabs are then encased in cladding of mild steel. The cladding is produced in the form of a frame with separate cover plates. The opening in the frame is dimensioned so that a slab is sliding fit in the opening. The thickness of the frame is about the same as the initial thicknes of the slab. The plates are typically 12 gauge sheet. One plate is seal welded to the frame. Then the coated slab is inserted and the otherplate is seal welded. l

The composite pack is then reduced by hot rolling.

FIG. 1 is a diagrammatic view of apparatus used in producing the alloy with which this invention is practiced;

FIG. 2 is an exploded view showing the cladding parts of the pack according to this invention; and

FIGS. 30, b, c, d, e and f are photo-micrographs at 200 magnification of cuts of sheets produced in the practice of this invention.

The apparatus shown in FIG. 1 :is a vacuum furnace 11 which typically may be bought from Stokes Vacuum Furnace Co. This furnace l1 includesa crucible 13 in the base of whichthere is a can 15 containing powder 17 enriched to 90% B-10. The can may be composed of mild steel and may contain typically about 3 pounds of powder. Upon the can, but within the crucible rods The pack is repeatedly heated to 1 125C. i C. and

then rolled.

The following Table IV presents a flow diagram of typical practice of the process.

TABLE IV Process Flow Diagram Receipt oi Boron Weigh Chemistry Sample Charge Makeup 0 an Mlt andlCast Clean Clean Clean Remelt Scrap Rernelt Scrap 1/ \1 Chemistry Metallography Study of typical sheets produced in this way has shown them to be free of end cracks and to have uniform boron distribution throughout. Contamination of the shop by boron during rolling is eliminated.

19 of stainless steel are disposed. Typically A181 304 or 304L steel may be used. The rods 1.9 may have a weight of 150 pounds.

The crucible may be disposed in an induction heater 21 having induction heating coils 23embedded in insu- Receipt oi Certified 304 stainless Sample Chemistry 4-- Mold Prep Chemistry Jacket Fabrication Zirconia Wash lation. The heating coils 23tnay be energized from an INDUCIOTHERM Kw motor generator set delivering power at about 4000 cycles per second. The furnace 21 and its contents are disposed in a chamber 25 which may be evacuated by adequate pumping equipment not shown.

In producing the boron stainless steel the following typical procedure is followed:

a. The furnace chamber 11 is evacuated to microns. b. The chamber is backfilled with argon to a negative 5 inches of mercury. c. The power is applied and slowly increased l kw per minute) to prevent thermal shock to crucible.

Maximum power is approximately 55 kw. Melting temperature is approximately l250C. Time for complete melting is about 1 hour after reaching full power.

d. After melting, the temperature is increased to a. Add one part polyvinyl alcohol (PVA) to 50 parts water.

b. Heat until the water comes to a slow boil c. Stir until all the PVA is dissolved (1. Mix the PVA-water solution with approximately 2-micron mesh zirconia. The amount of zirconia added is determined by the specific use for which it is intended, i.e., for smooth surfaces it is mixed so it can be brushed evenly; for filling large cracks, etc., the coating is mixed with a higher zirconia to liquid ratio.

e. Apply the coating. Various application techniques can be used such as brushing, spraying, or dipping.

f. Dry the coating part at 400F. 1 50F. for a minimum of 2 hours.

g. The slab 40 should have a white surface after application of the wash and drying.

Prior to rolling, the jacketed billets 40 are given a furnace soak for one-half hour at llC. in an impure argon atmosphere. The plates are then removed from the furnace and rolled on a Waterbury Farrel, two high, 200,000 lb. capacity mill with 14 in. diameter X 22 in. rolls as per the typical rolling schedule in the following 25 Table V.

TABLE V Includes cover plates MinuteszSeconds Thickness Thickness Pass in out 7: Time in Time out No. (in.) (in.) Reduction Furnace of Furnace Furnace Soak 1:40 2:10 1 1.400 1.200 14 2:13 2:20 2 1.200 1.000 16.5 2:23 2:31 3 1.000 0.800 20.0 Double Pass 4 0.800 0.700 12.5 2:35 2:42 5 0.700 0.600 14.3 Double Pass 6 0.600 0.500 16.7 2:48 2:55 7 0.500 0.450 10.0 2:58 3:05 8 0.450 0.420 6.7 3:08 3:15

Total Elapsed Time 1 hour minutes Total Reduction 70% Actual Plate Thickness after Removal of the Jacket 0.350 to 0.360"

i. After cooling, the mold is unbolted and stripped The rolling speed is --75 fpm. The major problem in from the casting. j. The cast billets are attached to the hot top by means of a A inch riser which is easily broken. During the melting the furnace is stirred inductively.

After mold stripping, the billets were sand-blasted. All burrs were ground off. Any surface porosity is ground smooth. It has been found that sand-blasted billets resulted in a better as-rolled finish than billets that had not been sand blasted.

Chemistry samples were taken from the top and bottom of one plate in each melt and submitted to chemistry for total boron analysis.

The billets were then coated with zirconia, dried, and welded into a mild steel jacket which had also been coated with zirconia. The zirconia formed an effective reaction barrier between the billet and jacket material.

rolling is maintaining the proper rolling temperature. Severe edge cracking occurs if the plate temperature is above 1165C. or below -1075C.

After rolling, the plate is air cooled and then stripped from the jacket. The zirconia coating on the plate prevents any reaction between the plate and the cladding and consequently the cladding is readily removed. Stripping is accomplished by clamping the plate in a vice and bending the mild steel side strip until the cover plate fractures.

3(0) and (d) and 3(e) and (f) are respectively like photomicrographs of cuts from the opposite ends of other plates. The dark dots correspond to boron or boride and are seen to be uniformly distributed throughout the graphs revealing that the boron is uniform throughout the alloy.

While preferred embodiments have been disclosed herein many modifications thereof are feasible. This invention then is not to be restricted except insofar as is necessitated by the spirit of the prior art.

I claim:

1. The method of reducing a billet of stainless steel characterized by substantial quantities of chromium and also containing substantial quantities, of the order of 2%, of boron to a substantially smaller thickness, which comprises covering the billet with a thin coating of a protective material, completely enclosing said billet so coated in a jacket of a material which retains its strength at temperatures between 1100C. and 1200C., heating the package consisting of the billet and jacket to a temperature of about 1 100C., and rolling said package until the billet has the desired thickness, the protective material preventing adhesion of the billet to the jacket material.

2. Themethod of claim 1 wherein the billet contains up to 2.3 to 2.5% boron, the jacket is composed of mild carbon steel and the coating material is zirconium oxide.

3. The method of claim 1 wherein the stainless steel consists essentially of the following composition in percent by weight:

Boron 1.87 to 2.04 Carbon .12 to .15 Manganese 1.35 to 1.53 Silicon .54 to .65 Nickel 8.70 to 9.25 Chromium 17.13 to 17.60 Sulphur Not more than .016 Phosphorous Not more than .017 Iron Remainder.

nately boron 10. 

2. The method of claim 1 wherein the billet contains up to 2.3 to 2.5% boron, the jacket is composed of mild carbon steel and the coating material is zirconium oxide.
 3. The method of claim 1 wherein the stainless steel consists essentially of the following composition in percent by weight:
 4. The method of claim 1 wherein the temperature of the billet is maintained between about 1075*C. and about 1165*C. to avoid edge cracking of the billet.
 5. The method of claim 1 for reducing strainless steel containing about 17% chromium.
 6. The method of claim 1 for reducing stainless steel to make control rods for use in the corrosive surroundings of a reactor and wherein the boron is predominately boron
 10. 